The present invention relates to an optical transmission line which is used when, for example, a wavelength division multiplexed optical transmission is carried out.
The amount of communication information has tended to increase dramatically due to the development of the information society. Along with the increase of information, the wavelength division multiplexed transmission (WDM transmission) is widely recognized in the communication field and now the era of the wavelength division multiplexed transmission has arrived. In a wavelength division multiplexed transmission, light with a plurality of wavelengths can be transmitted in a single optical fiber. Therefore, the wavelength division multiplexed transmission is an optical transmission system which is suitable for large capacity high speed communication and, at present, this transmission technology is being vigorously researched.
As is widely known in the art, a single mode optical fiber, having a zero-dispersion within the wavelength band in the vicinity of the wavelength of 1.3 xcexcm, has been established on a global scale as the transmission network for optical communication. However, in the case that the previously established single mode optical fiber having a zero-dispersion in the vicinity of 1.3 xcexcm is utilized and wavelength division multiplexed transmission is carried out by using the wavelength band in the vicinity of 1.3 xcexcm, the 1.55 xcexcm wavelength band, which is the gain band of a conventional optical amplifier, and the wavelength band do not agree with each other. Therefore, the problem arises that a conventional optical amplifier cannot be utilized for the wavelength division multiplexed transmission which uses the above described single mode optical fiber and, subsequently, long distance optical communication becomes difficult. Here, the above used term, xe2x80x9c1.55 xcexcm wavelength band,xe2x80x9d means a wavelength band of which the center is approximately the wavelength 1550 nm, such as from 1530 nm to 1570 nm, and hereinafter the term, the 1.55 xcexcm wavelength band, is used with this meaning.
Therefore, recently a system for carrying out an optical transmission by using a dispersion shift optical fiber, of which the zero-dispersion wavelength is shifted from the vicinity of 1.3 xcexcm to the vicinity of 1.55 xcexcm, and the above described optical amplifier has been proposed in order to solve the above described problem. When an optical signal is transmitted at a wavelength in the vicinity of 1.55 xcexcm by using a dispersion shift optical fiber having a zero-dispersion in the wavelength in the vicinity of 1.55 xcexcm, a signal light is amplified by the optical amplifier and a signal transmission becomes possible with little waveform distortion by dispersion.
However, while research of wavelength division multiplexed transmission technology has progressed, light signals have become of a higher power and, in the case that a dispersion shift optical fiber is used for the wavelength division multiplexed transmission, a non-linearity phenomenon due to the mutual action between each signal wave arises as a new problem. Therefore, a dispersion shift optical fiber which controls the above described wavelength dispersion and dispersion slope and which makes it possible to lower the non-linearity phenomenon is desirable as a dispersion shift optical fiber for the wavelength division multiplexed transmission.
Concerning the study for the solving of the non-linearity phenomenon, research for controlling a four light wave mixture has always been vigorous. The four light wave mixture greatly influences waveform distortion and, therefore, it is important to control this four light wave mixture. As an example of the study of four light wave mixture control, academic paper OFC ""94 Technical Digest PD19, for example, reports a dispersion shift optical fiber of which the zero-dispersion wavelength is shifted from the signal light wavelength in order to control the four light wave mixture.
When an optical fiber for optical transmission has a zero-dispersion in the signal light wavelength band, a four light wave mixture can easily be produced. Therefore, the above described paper reports that control of the four light wave mixture is possible by allowing the dispersion shift optical fiber used for the optical transmission to have a microscopic dispersion at the wavelength of 1.55 xcexcm, which is the signal light wavelength. Here, the above described microscopic dispersion is a dispersion of which the absolute value of, for example, the local dispersion (a dispersion per unit length) is approximately 2 to 3 ps/nm/km.
Since the waveform distortions by the SPM (Self-phase Modulation) or the XPM (Cross-phase Modulation) occurring in the above described non-linearity phenomenon has become a serious problem, there has recently been much research into controlling those waveform distortions. As for a means to solve this problem, the academic report OFC ""97 TuN1b, or the like, report research aimed at limiting the non-linearity refractive index (n2) to a small value. Moreover, research aimed at making this non-linearity refractive index a small value and research aimed at making the effective core section area of the dispersion shift optical fiber (Aeff) a large value have drawn attention. The distortion xcfx86NL of the signal through the non-linearity phenomenon is, in general, represented by the following equation (1). Therefore, when the effective core section area of the optical fiber is large the waveform distortion of the signal through the non-linearity phenomenon can be made small.
xcfx86NL=(2xcfx80xc3x97n2xc3x97Leffxc3x97P)/(xcexxc3x97Aeff)xe2x80x83xe2x80x83(1) 
Here, in the equation (1), xcfx80 represents the circular constant, Leff represents the effective optical fiber length, P represents a signal light intensity and xcex represents a signal light wavelength, respectively.
The above described effective core section area is expressed by the following equation (2) by using a constant k and the mode field diameter (MFD) of the optical fiber. Therefore, the larger the mode field diameter is, the larger the effective core section area becomes and it is understood that low non-linearity can be achieved very effectively.
Aeff=kxc3x97(MFD)2xe2x80x83xe2x80x83(2) 
In this way the expansion of the mode field diameter and the expansion of the effective core section area in an optical fiber used for the wavelength division multiplexed transmission are very important and they have drawn a lot of attention. The expansion of the mode field diameter and the expansion of the effective core section area in an optical fiber used in the wavelength division multiplexed transmission are reported in the academic paper OFC ""96 WK15 and OFC ""97 YuN2.
It is known that the non-linearity phenomenon can be caused more easily when the signal light intensity inputted to the optical fiber is larger. Therefore, it is proposed in the Japanese Unexamined Patent Publication No. Hei-9(1997)-211511 that an optical transmission line be formed by connecting an optical fiber with high non-linearity to the emission end of an optical fiber with low non-linearity so that the light emitted from the optical transmission line is controlled so as not to cause distortion resulting from the non-linearity phenomenon. Here, this proposal describes that the waveform distortion by the dispersion is also controlled by making the symbols of the dispersion value mutually different within the 1.5 xcexcm wavelength band of the above described optical fiber forming the optical transmission line.
As proposed in the Japanese Unexamined Patent Publication No. Hei-9(1997)-211511, however, no concrete configuration or the like are shown with respect to the dispersion value of the optical fiber forming the optical transmission line and, instead, merely the configuration of the connection of the optical fiber with high non-linearity to the emission end of the optical fiber with low non-linearity is shown. From such a configuration only, though, it is difficult to form an optical transmission line which is able to control the distortion resulting from the non-linearity phenomenon and the distortion resulting from dispersion.
Here, in this proposal, it is possible to apply a dispersion shift optical fiber of which the dispersion value is approximately xc2x12 to 3 ps/nm/km within the wavelength of 1.55 xcexcm. In this case, the absolute value of the dispersion value within the wavelength of 1.55 xcexcm is extremely small and, therefore, there is no guarantee that the optical transmission line will receive no influence from the four light wave mixture. And, in this case, where it is attempted to carry out the wavelength division multiplexed optical transmission by using light with a wide range of wavelengths within the 1.5 xcexcm wavelength band, the dispersion in any wavelength within this range sometimes becomes very close to zero (for example, within xc2x10.5 ps/nm/km). Then, the optical transmission line receives influences from the four light wave mixture.
On the other hand, the single mode optical fiber is superior in terms of low non-linearity. Therefore, in order that this characteristic is utilized to control the waveform distortion resulting from the above described non-linearity phenomenon, a proposal is made that the optical transmission line be formed of a single mode optical fiber and a short dispersion compensation optical fiber is connected to the emission end of this optical transmission line. This proposal is made in, for example, the Japanese Unexamined Patent Publication No. Hei-6(1994)-11620, or the like. This proposal attempts to implement low non-linearity of the optical transmission line using the above described configuration and to control the wavelength dispersion of the single mode optical fiber.
However, the mode field diameter within the 1.5 xcexcm wavelength band of the dispersion compensation optical fiber becomes, in design, too small to compensate for the dispersion characteristics of the single mode optical fiber with short length, and easily causes the non-linearity phenomenon. Therefore, in the scheme of the above described Japanese Unexamined Patent Publication No. Hei-6(1994)-11620 the problem of the non-linearity phenomenon cannot be controlled.
In addition, the above described dispersion compensation optical fiber has an extremely large absolute value of the dispersion value within the wavelength of the 1.55 xcexcm band. Therefore, the optical transmission line formed by connecting the single mode optical fiber and the dispersion compensation optical fiber has an extremely large absolute value of local dispersion (dispersion value per unit length) on the side of the dispersion compensation optical fiber. Accordingly, this optical transmission line cannot completely control the waveform distortion resulting from the dispersion even though the wavelength dispersion of the entire optical transmission line can be made approximately zero and, therefore, there is a risk that the waveform distortion resulting from dispersion might remain.
In addition, recently an optical transmission line has been proposed where a dispersion compensation optical fiber, which has dispersion characteristics opposite to those of the single mode optical fiber, is connected to a single mode optical fiber of the same length. This proposal is made in the academic paper ECOC ""97 Vol. P127. The dispersion compensation optical fiber used for the optical transmission line of this proposal has low non-linearity compared to the above described dispersion compensation optical fiber which is short and compensates for the dispersion of the single mode optical fiber.
On the other hand, the dispersion value within the wavelength of 1.55 xcexcm band of the single mode optical fiber is approximately 17 ps/nm/km and, in order to prevent the influence of a local dispersion the absolute value of the dispersion value, needs to be made even smaller. However, as for optical fibers of which the dispersion is lower than that of the single mode optical fiber, only the dispersion shift optical fiber, which has an extremely small dispersion of which the dispersion value is within xc2x15 ps/nm/km, is known. And this dispersion shift optical fiber cannot control the non-linearity phenomenon as described above.
In addition, recently it has become required to further increase the amount of information communicated. Taking this into account, when the wavelength division multiplexed transmission is carried out by only using the 1.55 xcexcm wavelength band, there is a limit to the number of wavelengths which can be sent, which eventually causes saturation at a certain point. Therefore, a new optical transmission line is required which can make the 1.5 xcexcm wavelength band a utilizable wavelength band by expanding the utilizable wavelength band for the wavelength division multiplexed transmission to include the wavelength bands on both sides of the conventional 1.55 xcexcm wavelength band (for example, 1530 to 1570 nm). Here, the 1.5 xcexcm wavelength band denotes a wavelength band including the conventional 1.5 xcexcm wavelength band, such as 1520 to 1620 nm, and hereinafter the term 1.5 xcexcm band is used in this sense.
The present invention is provided to solve the above described conventional problems. The purpose of the present invention is to provide an optical transmission line which has the characteristics as shown the following. That is to say, the purpose of the present invention is firstly, to make the dispersion of the entire optical transmission line approximately zero when the optical transmission line according to the present invention is used for the wavelength division multiplexed transmission and, secondly, to control local dispersion of the optical fiber which forms the optical transmission line and, thereby, to control the waveform distortion resulting from dispersion with almost no failure and, thirdly, to make possible a high quality signal light transmission which can control the waveform distortion resulting from the non-linearity phenomenon.
In order to achieve the above described purposes, the present invention provides the means for solving the problems with the configurations as follows: that is to say, the first configuration of the present invention is characterized in that said optical transmission line is formed by connecting, in series, a first optical fiber of which the dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band is 6 to 14 ps/nm/km and a second optical fiber of which the dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band is xe2x88x9214 to xe2x88x926 ps/nm/km and in that the dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band is approximately zero for the entire optical transmission line.
The second configuration of the present invention is, in addition to the above first configuration, characterized in that the closer to the input end of an optical signal the arrangement position of the optical fiber is, the lower the non-linearity of the optical fiber is.
The third configuration of the present invention is, in addition to the above first or second configuration, characterized in that the dispersion slope of the first optical fiber is of the opposite symbol to the second optical fiber, and in that the dispersion slope in the set wavelength band within the 1.5 xcexcm wavelength band is approximately zero for the entire optical transmission line
The fourth configuration of the present invention is, in addition to the above first or second configuration, characterized in that the characteristics of the first optical fiber in the wavelength in the vicinity of the center of the set wavelength band within the 1.5 xcexcm wavelength band are as follows: the transmission loss is 0.25 dB/km or less, the polarized wave mode dispersion value is 0.15 ps/km1/2 or less, the bending loss with the bending diameter of 20 mm is 10 dB/m or less and the mode field diameter is 9.5 xcexcm or more.
The fifth configuration of the present invention is, in addition to the above third configuration, characterized in that the characteristics of the first optical fiber in the wavelength in the vicinity of the center of the set wavelength band within the 1.5 xcexcm wavelength band are as follows: the transmission loss is 0.25 dB/km or less, the polarized wave mode dispersion value is 0.15 ps/km1/2 or less, the bending loss with the bending diameter of 20 mm is 10 dB/m or less and the mode field diameter is 9.5 xcexcm or more.
The sixth configuration of the present invention is, in addition to the above fourth configuration, characterized in that the characteristics of the first optical fiber in the set wavelength band within the 1.5 xcexcm wavelength band are as follows: the transmission loss is 0.25 dB/km or less, the polarized wave mode dispersion value is 0.15 ps/km1/2 or less, the bending loss with the bending diameter of 20 mm is 10 dB/m or less and the mode field diameter is 9.5 xcexcm or more.
The seventh configuration of the present invention is, in addition to the above fifth configuration, characterized in that the characteristics of the first optical fiber in the set wavelength band within the 1.5 xcexcm wavelength band are as follows: the transmission loss is 0.25 dB/km or less, the polarized wave mode dispersion value is 0.15 ps/km1/2 or less, the bending loss with the bending diameter of 20 mm is 10 dB/m or less and the mode field diameter is 9.5 xcexcm or more.
The eighth configuration of the present invention is, in addition to the above first or second configuration, characterized in that the first optical fiber is a single peak-type optical fiber which is formed by covering a core with a cladding and of which the refractive index distribution shape forms a profile of the xcex1th power.
The ninth configuration of the present invention is, in addition to the above third configuration, characterized in that the first optical fiber is a single peak-type optical fiber which is formed by covering a core with a cladding and of which the refractive index distribution shape forms a profile of the xcex1th power.
The tenth configuration of the present invention is, in addition to the above fourth configuration, characterized in that the first optical fiber is a single peak-type optical fiber which is formed by covering a core with a cladding and of which the refractive index distribution shape forms a profile of the xcex1th power.
The eleventh configuration of the present invention is, in addition to the above fifth configuration, characterized in that the first optical fiber is a single peak-type optical fiber which is formed by covering a core with a cladding and of which the refractive index distribution shape forms a profile of the xcex1th power.
The twelfth configuration of the present invention is, in addition to the above first or second configuration, characterized in that the first optical fiber is a step-type optical fiber, which is formed by covering a center core with a side core of which the refractive index is smaller than that of said center core, and by covering said side core with a cladding of which the refractive index is smaller than that of said side core.
The thirteenth configuration of the present invention is, in addition to the above third configuration, characterized in that the first optical fiber is a step-type optical fiber, which is formed by covering a center core with a side core of which the refractive index is smaller than that of said center core, and by covering said side core with a cladding of which the refractive index is smaller than that of said side core.
The fourteenth configuration of the present invention is, in addition to the above fourth configuration, characterized in that the first optical fiber is a step-type optical fiber, which is formed by covering a center core with a side core of which the refractive index is smaller than that of said center core, and by covering said side core with a cladding of which the refractive index is smaller than that of said side core.
The fifteenth configuration of the present invention is, in addition to the above fifth configuration, characterized in that the first optical fiber is a step-type optical fiber, which is formed by covering a center core with a side core of which the refractive index is smaller than that of said center core, and by covering said side core with a cladding of which the refractive index is smaller than that of said side core.
The sixteenth configuration of the present invention is, in addition to the above first or second configuration, characterized in that the first optical fiber is a depressed center core-type optical fiber which is formed by covering a center core with a side core of which the refractive index is larger than that of said center core and by covering said side core with a cladding of which the refractive index is smaller than that of said side core and larger than that of said center core.
The seventeenth configuration of the present invention is, in addition to the above third configuration, characterized in that the first optical fiber is a depressed center core-type optical fiber which is formed by covering a center core with a side core of which the refractive index is larger than that of said center core and by covering said side core with a cladding of which the refractive index is smaller than that of said side core and larger than that of said center core.
The eighteenth configuration of the present invention is, in addition to the above fourth configuration, characterized in that the first optical fiber is a depressed center core-type optical fiber which is formed by covering a center core with a side core of which the refractive index is larger than that of said center core and by covering said side core with a cladding of which the refractive index is smaller than that of said side core and larger than that of said center core.
The nineteenth configuration of the present invention is, in addition to the above fifth configuration, characterized in that the first optical fiber is a depressed center core-type optical fiber which is formed by covering a center core with a side core of which the refractive index is larger than that of said center core and by covering said side core with a cladding of which the refractive index is smaller than that of said side core and larger than that of said center core.
The twentieth configuration of the present invention is, in addition to the above first or second configuration, characterized in that the first optical fiber is an optical fiber which is formed by covering a center core with a first side core, by covering said first side core with a second side core and by covering said second side core with a cladding and which satisfies xcex942 greater than xcex943 greater than xcex941 when the relative refractive index difference of said center core for said cladding is xcex941, the relative refractive index difference of said first side core for said cladding is xcex942, and the relative refractive index difference of said second side core for said cladding is xcex943.
The twenty-first configuration of the present invention is, in addition to the above third configuration, characterized in that the first optical fiber is an optical fiber which is formed by covering a center core with a first side core, by covering said first side core with a second side core and by covering said second side core with a cladding and which satisfies xcex942 greater than xcex943 greater than xcex941 when the relative refractive index difference of said center core for said cladding is xcex941, the relative refractive index difference of said first side core for said cladding is xcex942, and the relative refractive index difference of said second side core for said cladding is xcex943.
The twenty-second configuration of the present invention is, in addition to the above fourth configuration, characterized in that the first optical fiber is an optical fiber which is formed by covering a center core with a first side core, by covering said first side core with a second side core and by covering said second side core with a cladding and which satisfies xcex942 greater than xcex943 greater than xcex941 when the relative refractive index difference of said center core for said cladding is xcex941, the relative refractive index difference of said first side core for said cladding is xcex942, and the relative refractive index difference of said second side core for said cladding is xcex943.
The twenty-third configuration of the present invention is, in addition to the above fifth configuration, characterized in that the first optical fiber is an optical fiber which is formed by covering a center core with a first side core, by covering said first side core with a second side core and by covering said second side core with a cladding and which satisfies xcex942 greater than xcex943 greater than xcex941 when the relative refractive index difference of said center core for said cladding is xcex941, the relative refractive index difference of said first side core for said cladding is xcex942, and the relative refractive index difference of said second side core for said cladding is xcex943.
The twenty-fourth configuration of the present invention is, in addition to the above first or second configuration, characterized in that the characteristics of the second optical fiber in the wavelength in the vicinity of the center of the set wavelength band within the 1.5 xcexcm wavelength band are as follows: the transmission loss is 0.30 dB/km or less, the polarized wave mode dispersion value is 0.15 ps/km1/2 or less, the bending loss with the bending diameter of 20 mm is 10 dB/m or less and the mode field diameter is 5.5 xcexcm or more.
The twenty-fifth configuration of the present invention is, in addition to the above third configuration, characterized in that the characteristics of the second optical fiber in the wavelength in the vicinity of the center of the set wavelength band within the 1.5 xcexcm wavelength band are as follows: the transmission loss is 0.30 dB/km or less, the polarized wave mode dispersion value is 0.15 ps/km1/2 or less, the bending loss with the bending diameter of 20 mm is 10 dB/m or less and the mode field diameter is 5.51 xcexcm or more.
The twenty-sixth configuration of the present invention is, in addition to the above twenty-fourth configuration, characterized in that the characteristics of the second optical fiber in the set wavelength band within the 1.5 xcexcm wavelength band are as follows: the transmission loss is 0.30 dB/km or less, the polarized wave mode dispersion value is 0.15 ps/km1/2 or less, the bending loss with the bending diameter of 20 mm is 10 dB/m or less and the mode field diameter is 5.5 xcexcm or more.
The twenty-seventh configuration of the present invention is, in addition to the above twenty-fifth configuration, characterized in that the characteristics of the second optical fiber in the set wavelength band within the 1.5 xcexcm wavelength band are as follows: the transmission loss is 0.30 dB/km or less, the polarized wave mode dispersion value is 0.15 ps/km1/2 or less, the bending loss with the bending diameter of 20 mm is 10 dB/m or less and the mode field diameter is 5.5 xcexcm or more.
The twenty-eighth configuration of the present invention is, in addition to the above first or second configuration, characterized in that the second optical fiber is a W-type optical fiber which is formed by covering a center core with a side core of which the refractive index is smaller than that of said center core and by covering said side core with a cladding of which the refractive index is larger than that of said side core and smaller than that of said center core.
The twenty-ninth configuration of the present invention is, in addition to the above third configuration, characterized in that the second optical fiber is a W-type optical fiber which is formed by covering a center core with a side core of which the refractive index is smaller than that of said center core and by covering said side core with a cladding of which the refractive index is larger than that of said side core and smaller than that of said center core.
The thirtieth configuration of the present invention is, in addition to the above twenty-fourth configuration, characterized in that the second optical fiber is a W-type optical fiber which is formed by covering a center core with a side core of which the refractive index is smaller than that of said center core and by covering said side core with a cladding of which the refractive index is larger than that of said side core and smaller than that of said center core.
The thirty-first configuration of the present invention is, in addition to the above twenty-fifth configuration, characterized in that the second optical fiber is a W-type optical fiber which is formed by covering a center core with a side core of which the refractive index is smaller than that of said center core and by covering said side core with a cladding of which the refractive index is larger than that of said side core and smaller than that of said center core.
The thirty-second configuration of the present invention is, in addition to the above first or second configuration, characterized in that the second optical fiber is an optical fiber which is formed by covering a center core with a first side core, by covering said first side core with a second side core and by covering said second side core with a cladding and which satisfies xcex941 greater than xcex943 greater than xcex942 when the relative refractive index difference of said center core for said cladding is xcex941, the relative refractive index difference of said first side core for said cladding is xcex942, and the relative refractive index difference of said second side core for said cladding is xcex943.
The thirty-third configuration of the present invention is, in addition to the above third configuration, characterized in that the second optical fiber is an optical fiber which is formed by covering a center core with a first side core, by covering said first side core with a second side core and by covering said second side core with a cladding and which satisfies xcex941 greater than xcex943 greater than xcex942 when the relative refractive index difference of said center core for said cladding is xcex941, the relative refractive index difference of said first side core for said cladding is xcex942, and the relative refractive index difference of said second side core for said cladding is xcex943.
The thirty-fourth configuration of the present invention is, in addition to the above twenty-fourth configuration, characterized in that the second optical fiber is an optical fiber which is formed by covering a center core with a first side core, by covering said first side core with a second side core and by covering said second side core with a cladding and which satisfies xcex941 greater than xcex943 greater than xcex942 when the relative refractive index difference of said center core for said cladding is xcex941, the relative refractive index difference of said first side core for said cladding is xcex942, and the relative refractive index difference of said second side core for said cladding is xcex943.
The thirty-fifth configuration of the present invention is, in addition to the above twenty-fifth configuration, characterized in that the second optical fiber is an optical fiber which is formed by covering a center core with a first side core, by covering said first side core with a second side core and by covering said second side core with a cladding and which satisfies xcex941 greater than xcex943 greater than xcex942 when the relative refractive index difference of said center core for said cladding is xcex941, the relative refractive index difference of said first side core for said cladding is xcex942, and the relative refractive index difference of said second side core for said cladding is xcex943.
Here, in the present invention, xe2x80x9cset wavelength bandxe2x80x9d means a wavelength band having at least a 30 nm band and xe2x80x9cwavelength in the vicinity of the center of the set wavelength bandxe2x80x9d means a wavelength in a range within 5 nm from the center wavelength of the set wavelength band.
In the above described configuration of the present invention, both of the first and the second optical fibers making up the optical transmission line have the absolute value of the dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band which is 6 ps/nm/km or more and the dispersion value in the set wavelength within the 1.5 xcexcm wavelength band is shifted from zero. Therefore, the optical transmission line of the present invention can control the generation of the four light wave mixture which is supposed to dramatically influence the waveform distortion resulting from the non-linearity phenomenon and can control the waveform distortion resulting from the non-linearity phenomenon.
In both of the above described first and second optical fibers, the absolute value of the dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band is 14 ps/nm/km or less and the absolute value of the dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band is small compared to a single mode optical fiber or the like. Therefore, the optical transmission line of the present invention can control a local dispersion in the optical fiber making up the optical transmission line and can control the waveform distortion resulting from the local dispersion.
And when the dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band is too great, the waveform distortion resulting from the wavelength dispersion becomes large. In both of the above described first and second optical fibers applied to the optical transmission line of the present invention, however, the absolute value of the dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band is 14 ps/nm/km or less. That is to say, in the above described first and second optical fibers, the absolute value of the dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band is smaller compared to a single mode optical fiber or the like. Therefore, the optical transmission line of the present invention can control a local dispersion in the first and the second optical fibers and can control the waveform distortion resulting from the local dispersion.
And, since the dispersion value (total dispersion value) in the set wavelength band within the 1.5 xcexcm wavelength band for the entire optical transmission line according to the present invention is made approximately zero, residual dispersion hardly exists in the entire optical transmission line and the distortion of the signal waveform resulting from the residual dispersion can be controlled.
In particular, the higher the intensity of the incoming light inputted to the optical fiber is, the more likely the above described non-linearity phenomenon will be generated. Therefore, in the optical transmission line according to the present invention, where the closer to the input end of the optical signal the arrangement location of the optical fiber is the lower the non-linearity of the optical fiber is, control of the waveform distortion resulting from the non-linearity phenomenon can be further assured. And when the mode field diameter in the set wavelength band within the 1.5 xcexcm wavelength band is made to be large the influence by the self-phase modulation or by the cross-phase modulation, or the like, of the non-linearity phenomenon can be controlled and the waveform distortion resulting from the non-linearity phenomenon can be further controlled in a more sure fashion.
In addition, when the transmission loss in the set wavelength band within the 1.5 xcexcm wavelength band is 0.30 dB/km or less (0.25 dB/km or less in the first optical fiber), the polarized wave mode dispersion value in the set wavelength band within the 1.5 xcexcm wavelength band is 0.15 ps/km1/2 or less, and the bending loss with the bending diameter of 20 mm in the set wavelength band within the 1.5 xcexcm wavelength band is 10 dB/m or less, the waveform distortion resulting from the polarized wave mode dispersion can be controlled to gain an excellent optical transmission line with small transmission loss or bending loss.
In addition, the configuration where the first optical fiber is a single peak-type optical fiber, the configuration of a step type optical fiber and a configuration of a depressed center core type optical fiber can optimize the refractive index profile of the first optical fiber due to those refractive index profiles. Therefore, as for this configuration, the above described optical transmission line with excellent effects can be formed by utilizing the first optical fiber with the above described refractive index profile.
In addition, the configuration of the first optical fiber which satisfies xcex942 greater than xcex943 greater than xcex941 in the relationship among the relative refractive index difference xcex941 for the center core cladding, the relative refractive index difference xcex942 for the cladding of the first side core covering the center core and the relative refractive index difference xcex943 for the cladding of the second side core covering the first side core has the same effects as the configuration of the first optical fiber by the above described single peak-type optical fiber, or the like.
In addition, the configuration where the second optical fiber is a W type optical fiber optimizes the refractive index profile of the second optical fiber due to this refractive index profile and the optical transmission line with the above described excellent effects can be formed by using the second optical fiber of the above described refractive index profile.
In addition, the configuration of the second optical fiber which satisfies xcex941 greater than xcex943 greater than xcex942 in the relationship among relative refractive index difference xcex941 for the center core cladding, the relative refractive index difference xcex942 for the cladding of the first side core covering the center core and the relative refractive index difference xcex943 for the cladding of the second side core covering the first side core also has the same effects as the above.